1. Field of the Invention
This invention generally relates to a filter using a film bulk acoustic resonator (hereinafter referred to as FBAR) used in mobile communications and high-frequency radio frequency communications such as cellular phones, PHS and wireless LAN. More specifically, the present invention relates to a duplexer equipped with a filter composed of multiple FBARs arranged and connected in a ladder form.
2. Description of the Related Art
Recently, there has been considerable activity in the development of filter elements used for high frequency communications and capable of allowing electric signals in a specific frequency range to pass by the combination of SAW or BAW resonators having a piezoelectric material (SAW is an abbreviation of surface acoustic wave, and BAW is an abbreviation of bulk acoustic wave). The filter devices that utilize SAW or BAW have a small outer size and a sharp roll-off characteristic, as compared to dielectric filters or ceramic filters, and are thus suitable for components for mobile communications such as portable phones, which are required to have a compact size and a wide comparative bandwidth. The SAW or BAW filter as described above may be typically a ladder-type filter in which resonators are arranged and connected in a ladder form. An exemplary ladder-type filter is illustrated in FIG. 1.
As is well known, the insertion loss and the out-of-band attenuation of the ladder-type filter can easily be controlled by simply changing the number of stages of the ladder form and the capacitance ratio of resonators in series and parallel arms. Further, the design sequence of the ladder-type filter is simple. From these viewpoints, the ladder-type filter is widely used. Japanese Patent No. 2800905 (Document 1) discloses a method of more flexibly changing the performance of the ladder-type filter. This method adds an inductance to a resonator in parallel or series. In the practical use of SAW or BAW, the inductance is implemented by a conductive pattern on an exposed or inner surface of a multilayered substrate or a wire. FIGS. 2 and 3 are equivalent circuits of filters having an additional inductance described above.
A duplexer is an applied component of the ladder-type filter of SAW or BAW. The duplexer is used for a communication apparatus having the functions of simultaneously sending and receiving signals at different frequencies, such as a CDMA system. The duplexer functions to suppress interference between the transmitted and received signals and allow only signals in a required frequency range to pass therethrough. Generally, the duplexer has an arrangement such that a transmit filter, a receive filter and a phase matching circuit are integrated into a single component. The transmit and receive filters are used as band-pass filters that allow the respective signals to pass therethrough, and are connected to an antenna. In this case, the transmitted and received signals may be interfered. The phase shifter, which functions as a phase matching circuit, is used to cope with the interference. The phase shifter is an electrical element having a combination of an inductance and a capacitance in the equivalent circuit. Japanese Patent No. 2905094 (Document 2) discloses a duplexer using SAW or BAW in which a distributed-constant line of a conductive pattern formed in a multilayered substrate is used as a phase shifter, as shown in FIG. 4. Japanese Patent No. 3487692 (Document 3) or Japanese Patent Application Publication No. 2001-127588 (Document 4) discloses a combination of inductance and capacitance elements of the lumped parameter type such as a wound coil, chip inductor and a chip capacitor, which may be used along with a substrate or cap.
However, the conventional techniques have the following problems. As described in Document 3, the method disclosed in Document 1 uses the conductive pattern for the phase line, which occupies a two- or three-dimensional space. For example, a duplexer for the WCDMA band in a frequency range of 2 GHz requires two conductive patterns which are 0.6 mm thick and 25 mm long in order to shift the phases of the transmit and receive filters by 180 degrees with a conductive pattern that is made of alumina having a dielectric constant of 9.5 and a width of 0.1 mm and has a characteristic impedance of 50 Ω. As has been described, the additional inductance is generally used to improve the performance of the ladder-type filter. When the additional inductance is implemented by a conductive pattern, it is necessary to separate the conductive patterns from each other at a sufficient distance because a close arrangement of the conductive patterns results in coupling of the inductances and degrades the desired filter characteristics. However, the separate arrangement of the conductive patterns does not realize compact packaging of the duplexer.
The technique of realizing the phase shifter with inductance and capacitance of the lumped parameter type as described in Documents 2 and 3 may realize the compact phase shifter as compared to the phase shifter with the conductive patterns if the chip inductor, wound coil and/or chip capacitor are used as discussed in Documents 2 and 3. However, even when the 1005-size multi-purpose chip components (1.0×0.5×0.5 [mm]) that are currently available are used, the package has a size as large as 8×5.1×2.5 [mm] as described in Document 2. It cannot be said that the above size is small because the duplexers of 5 mm□ or 8 mm□ are currently available. In addition, the number of chips increases by the number of passive components, and the production cost increases.